The present invention relates to hydrostatic radial piston machines deployed in power transmission and transaxle product and more particularly, is concerned with improving the layout of the hydrostatic components within the housing by way of a novel arrangement whereby such components can be located, mounted and supported in the surrounding housing structure.
Hydrostatic transmissions act as means for converting rotary mechanical motion, typically prosvided by an internal combustion engine, to fluid motion by means of a shaft driven hydraulic pump and where the pump is fluidly connected to a hydraulic motor, the volume and direction of the fluid delivery to the motor is governed by the displacement setting of the pump, and the motor is the mechanism for converting the fluid motion back to mechanical rotary motion. By regulating both the amount and direction of fluid motion between the pump and motor, usually achieved through the use of a variable-displacement pump, the output speed and rotational direction of the motor can be precisely controlled at a set input rotational speed of the pump. This device is useful in driving vehicles such as lawn and garden tractors or any apparatus requiring precise speed control. In many of such applications, the hydrostatic transmission is ideally integrated as part of a transaxle driving apparatus which further incorporates speed reducing means and, if the applications requires it, a differentialled axle of the type shown in U.S. Pat. No. 4,979,583 entitled Variable Speed Transaxle.
In the type of hydrostatic transmission employing radially arranged pistons, both pump and motor use cylinder-barrels which are preferably arranged to rotate on a common pintle-valve. Pistons are disposed within cylinders in both the pump and motor cylinder-barrels and protrude from their respective cylinders to be operatively connected to respective surrounding annular track-rings, and where the track-ring of the pump is adjustable in its position relative to the pintle-valve such that its eccentricity can be changed enabling the delivered fluid medium to be varied in both quantity and direction to the hydraulic motor. The track-ring of the hydraulic motor is normally held in a permanent eccentric position to the pintle-valve. Fluid delivered by the pump through the pintle-valve is thereby directed to the motor to enter each of the cylinder chambers in turn that are provided in the cylinder-barrel of the hydraulic motor. The eccentricity of the pump track ring can therefore be changed by the operator of the machine, and for example, this can be simply accomplished by pivoting the track-ring about an axis located at one end of the track ring, this axis define; generally being a pivot pin. In addition, a control mechanism is also provided so that the track-ring can swing or pivot about the axis of the pivot pin so that the eccentricity of the track-ring relative to the pintle-valve is alterable Such a hydrostatic transmission is shown in U.S. Pat. No. 5,078,659, entitled Variable Speed Transaxle, which is specifically incorporated herein by reference.
Hydrostatic transmissions of the type which are fully integrated in a transaxle housing structure which also contains within both speed reducing gearing and a mechanical differential are fast becoming the preferred driving apparatus for lawn tractors and the like. However, there are also many applications were the hydrostatic transmission is a "stand-alone" machine and this invention is applicable to both arrangements.
In the variable speed transaxle as disclosed in the '659 patent, the hydrostatic transmission is totally encapsulated inside a purposely formed internal chamber as defined by the surrounding housing structure of the transaxle, and comprises an input shaft driven hydraulic pump which is fluidly coupled to the hydraulic motor by means of a pintle-valve, the motor being connected through speed reduction gearing and differential to output axle shafts on which, in the case of a vehicle application, the wheels of the vehicle are attached. The transaxle housing structure is comprised of two housing elements which are fixed together along a horizontal parting-plane, also called the peripheral seam, the top element being the "cover" and the bottom element being the "case". The input drive shaft is supported by bearings in the cover and where a bevel pinion is attached to the shaft allowing the rotational drive axis to be displaced through ninety degrees by means of meshing with a complementary bevel geal attached to the cylinder-barrel of the pump. The bevel gear and cylinder-barrel assembly rotate about the cylindrical pintle-valve and where one or more saddle clamps are used to fixedly locate and position the pintle-valve onto part-cylindrical machined surfaces provided in the interior of the case. The pivot pin used to support the pump track-ring is held in place between respective pairs of part-cylindrical machined pockets in the case and cover, and the motor track-ring may also be held on pins as shown or alternatively, be held in place between machined pockets in the case and cover. As a result, the housing structure for such prior hydrostatic transmissions requires that both the case and cover be machined in order that the hydrostatic components can be accurately located in position. Furthermore, with this prior art construction, the loads acting on the hydrostatic components such as the pintle-valve, track-ring and pins as generated by the pressurised fluid medium acting on the pistons elements have to be absorbed directly into the surrounding housing structure through the associated machined pockets.
The present invention is a significant advance on the prior art hydrostatic transmissions of the type described above as it eliminates the requirement to machine the case element of the housing for location and support surfaces for the hydrostatic components. Furthermore, the invention provides a new solution whereby the force loads generated by the pressurised fluid medium acting on the piston elements are contained and absorbed within a framework rather than directly to the housing. As a result, thinner wall sections can be used in the housing allowing an attendant overall weight saving in the unit.
In a further embodiment of the invention, the framework structure is formed as part of a subsidiary housing member, and where the subsidiary housing member is machined such the comparatively larger sized cover and case housing members can be used in the die-cast state. This construction simplifies manufacture of the product in that all the machining operations can be carried out in the smallest of the three housing members, allowing the use of a smaller and less expensive CNC machiningcenters.